There have been many reports in the prior art relating to the general concept of providing direct transport of an agent which is toxic to tumor cells directly to tumors having .beta.-glucuronidase activity by conjugating the agent with glucuronic acid. Among such reports are Von Ardenne, M. et al. Agressologie, 1976, 17, 5, 261-264; East German Pat. No. 122,386; German Offenlegungsschrift No. 22 12 014; Sweeney et al, Cancer Research, 31, 477-478, April 1971; Baba et al, Gann, 69, 283-284, 1978; and Ball, C. R., Biochem. Pharm., 23, 3171-3177 (1974).
The Von Ardenne reference suggests broadly many types of aglycones which may be conjugated to glucuronic acid and will be active at the tumor site. These include, broadly, alkylating groups, antimetabolites, cytotoxins, membrane-active (lytic) groups, glycolysis stimulators, respiration inhibitors, inorganic and organic acids and cell cycle stoppers. The East German patent also suggests many such combinations including 5-fluorouracil-glucuronide, methotrexate-glucuronide, 6-mercaptopurene-glucuronide, aniline mustard-glucuronide and many others. The Offenlegungsschrift also mentions a large number of glucuronides. The Sweeney article relates to the anti-tumor activity of mycophenolic acid-.beta.-D-glucuronides, Baba relates to the anti-tumor activity of 5-fluorouracil-O-.beta.-D-glucuronide, and Ball relates to the anti-tumor activity of p-hydroxyaniline mustard glucuronide.
It has also been reported that the selectivity of this transport mechanism can be improved by hyperacidification of the tumor cells. The Von Ardenne reference supra, as well as the East German patent, clearly recognize the importance and the feasibility of hyperacidification of the tumor cells when using the glucuronide mechanism. The Von Ardenne reference speaks of a method that yields a pH difference of at least 1 pH unit and may therefore be used as a basis for selectivity. It refers to reaching steady state conditions after hyperacidification in which the brain pH is 7.0 and the tumor tissue pH is approximately 5.5 to 6.0. Note also Von Ardenne, M. et al, Pharmazie, 32 (2): 74-75, 1977.
Bicker, U., Nature, 252, Dec. 20-27, 1974, pp. 726-727, particularly notes that lysosomal enzyme .beta.-glucuronidase has an optimum pH of 5.2 and that for anti-tumor activity of glucuronides, the pH must be lowered such as by the administration of glucose. Experiments are detailed which indicate that the hyperacidification by glucose is necessary in order to obtain significant deconjugation of the glucuronides.
Even with hyperacidification of the tumor cells by known methods as, for example, glucose administration, however, there is still a problem in that other organs and tissues of the body which have a naturally occurring high .beta.-glucuronidase activity, will also release the toxic aglycones and thereby cause damage to healthy tissues. This is most particularly a problem with regard to the kidney which normally has an acid pH environment.
Another problem faced by the prior art is the fact that it is very difficult to synthesize glucuronides when the aglycone is a strong electron acceptor. This is because the glucuronide will become deconjugated (hydrolyzed) in the course of the classical process. U.S. Pat. No. 2,985,664 discloses a process for preparing a glucuronide in which the aglycone is a strong electron acceptor. However, the process suggested by this patent is not reproducable. Fenselau, C., Science, 198 (4317) 625-627, 1977, relates to a biosynthesis process for producing a glucuronide in which the aglycone is a strong electron acceptor. However, the problem still remains how to produce such a compound by chemical synthesis in a reproducible manner.